1. Field of the Invention
The present invention relates to a liquid crystal display device and, in particular, to a liquid crystal display device including a reflective display area which reflects incident external light for display and a transmissive display area which transmits light from the backside for display.
2. Description of the Related Art
Recently, the liquid crystal display device, which has the advantage of thin, light-weight, small, and low power consumption, is widely installed in and used for large size terminal devices such as monitors and televisions, middle size terminal devices such as laptop computers, cash dispensers, and vending machines, and small size terminal devices such as PDAs (Personal Digital Assistance), mobile telephones, and portable game devices.
In the liquid crystal display device, the liquid crystal molecules themselves do not emit light. Thus, some kinds of light are required for users to visually recognize the display. In general, the liquid crystal display devices can be roughly divided into a transmission type, a reflection type, and a semi-transmission type that uses both the transmitted light and the reflected light according to a type of a light source. The reflection type can utilize the external natural light for display so that low power consumption can be achieved. However, the display performance thereof such as the contrast level is inferior compared to the transmission type. Therefore, the transmission type and the semi-transmission type liquid crystal display devices are the mainstream nowadays.
The transmission type and the semi-transmission type liquid crystal display devices includes a light source provided at the back face, and use the light emitted from the light source to achieve display. In particular, medium or small size liquid crystal display devices are carried by users and used under various conditions. Therefore, the semi-transmission type liquid crystal display devices, which exhibit high visibility under any kinds of conditions through allowing users to visually recognize the reflective display at a bright place and the transmissive display at a dark place, are employed for the medium or small liquid crystal display devices.
The film compensated TN mode or the multi-domain vertical alignment mode has been used for the semi-transmission type liquid crystal display devices. However, it is proposed to apply the IPS (In-plane-switching) mode, which provides a wide viewing angle in principle, to the semi-transmission type.
FIG. 10 shows a liquid crystal display device of IPS mode that is disclosed in Japanese Patent Application Laid-open No. 2003-344837 (Patent Document 1). A semi-transmission type liquid crystal display device 1053 shown in FIG. 10A includes a lower substrate loll, an opposed side substrate 1012, a liquid crystal layer 1013 interposed between the lower substrate 1011 and the opposed side substrate 1012, a backlight 1028 placed underneath the lower substrate 1011, and polarizing plates 1021a, 1021b. 
The lower substrate 1011 includes a half wavelength plate 1029, a transparent insulating substrate 1022a, and an insulating film 1008a. The upper substrate 1012 includes an insulating film 1022b. 
The semi-transmission type liquid crystal display device 1053 includes a reflective area 1005 and a transmissive area 1006. The reflective area 1005 is an area that achieves display by reflecting external incident light, and the transmissive area 1006 is an area that achieves display by transmitting light from the backlight 1028.
In the reflective area 1005, a laminated body configured with insulating films 1008a, 1008b, a reflective plate 1009, and an insulating film 1008c is provided, and the liquid crystal layer 1013 is set to be in a thickness that corresponds to λ/4 light wavelength. In the transmissive area 1006, the thickness of the liquid crystal layer 1013 is set to be in a thickness that corresponds to λ/2 light wavelength. Further, a horizontal electric field driving electrode 1007, which is configured with a pixel electrode 1027 and a common electrode 1026 arranged in a lateral direction on the insulating film 1008c, is provided in the reflective area 1005. Furthermore, a horizontal electric field driving electrode 1007, which is configured with a pixel electrode 1027 and a common electrode 1026 arranged in a lateral direction on the insulating film 1008a, is provided in the transmissive area 1006.
As shown in FIG. 10B, when no voltage is applied between the common electrode 1026 and the pixel electrode 1027, the polarizing plate 1021b is arranged at an angle of 90 degrees and the liquid crystal layer 1013 is arranged at an angle of 45 degrees provided that the lower-side polarizing plate 1021a in the reflective area 1005 and the transmissive area 1006 is arranged at an angle of 0 degree. A twist angle of the liquid crystal layer 1013 is 0 degree. The half wavelength plate 1029 is arranged at an angle of 135 degrees.
As shown in FIG. 10C, when no voltage is applied between the pixel electrode 1027 and the common electrode 1026 in the reflective area 1005, linear polarized light arranged at an angle of 90 degrees which has passed through the polarizing plate 1021b becomes clockwise circular polarized light after passing through the liquid crystal layer 1013, and becomes counterclockwise circular polarized light after being reflected by the reflective plate 1009. Then, it becomes linear polarized light arranged at an angle of 0 degree after passing through the liquid crystal layer 1013. Therefore, the linear polarized light cannot emerge therefrom, thereby resulting in a black display.
When a voltage is applied between the pixel electrode 1027 and the common electrode 1026 in the reflective area 1005, the arranged angle of the liquid crystal layer 1013 is changed to 0 degree, so that the linear polarized light arranged at an angle of 90 degrees which has passed through the polarizing plate 1021b is reflected by the reflective plate 1009 as it is in the state of the linear polarized light even after passing through the liquid crystal layer 1013. Then, it passes through the liquid crystal layer 1013 again, and emerges therefrom as it is in the state of the linear polarized light arranged at an angle of 90 degrees, thereby resulting in a white display.
When no voltage is applied to the liquid crystal layer 1013 in the transmissive area 1006, linear polarized light arranged at an angle of 0 degree which has passed through the polarizing plate 1021a becomes linear polarized light arranged at an angle of 90 degrees after passing through the half wavelength plate 1029. When it passes through the liquid crystal layer 1013, the light rotates still further to be turned into linear polarized light arranged at an angle of 0 degree. The linear polarized light arranged at an angle of 0 degree cannot emerge from the polarizing plate 1021b arranged at an angle of 90 degrees, thereby resulting in a black display.
When a voltage is applied to the liquid crystal layer 1013 in the transmissive area 1006, the arranged angle of the liquid crystal layer 1013 is changed into 0 degree. In this state, linear polarized light arranged at an angle of 0 degree which has passed through the polarizing plate 1021a becomes linear polarized light arranged at an angle of 90 degrees after passing through the half wavelength plate 1029. The arranged angle of the linear polarized light is not rotated further even after passing though liquid crystal layer 1013. Therefore, the linear polarized light arranged at an angle of 90 degrees emerges from the polarizing plate 1021b arranged at an angle of 90 degrees, thereby resulting in a white display.
Next, structures of the pixel electrode and the common electrode disclosed in Patent Document 1 will be described by referring to FIG. 11. As shown in FIG. 11, in the reflective area 1005, a rough-surface reflective plate 1009 is formed on the insulating film 1008b, the planarizing film 1008c is formed on the rough-surface reflective plate 1009, and the pixel electrode 1027 and the common electrode 1026 are formed on the planarizing film 1008c with a transparent conductive film such as ITO.
However, the structures shown in FIG. 11 have such a problem that an edge of a resist cannot be exposed uniformly due to diffused reflection of the exposure light reflected by the rough-surface reflective plate and that the patterning accuracy becomes poor as a result, when the pixel electrode 1027 and the common electrode 1026 in the reflective area 1005 are patterned by the exposure. That is, when ITO is formed in the reflective area 1005 and it is exposed after applying a resist thereon, the exposure light is reflected by the rough-surface reflective plate 1009 after transmitting through the ITO, thereby exposing the resist from the back side as well. The rough-surface reflective plate 1009 is structured to diffusedly reflect the incident light from the front to suit the reflection display. Therefore, the reflected light reaches even to the unintended area of the resist, so that patterning cannot be performed as desired.